1. Field of the Invention
The present invention relates to a displacement machine for compressible media.
2. Description of the Related Art
A generic displacement machine based on the spiral principle is disclosed, for example, by DE-A-42 03 346. Machines of this type are distinguished by the fact that the delivery of the gaseous operating medium, consisting, for example, of air or an air/fuel mixture, has pulsations of relatively low amplitude, and can therefore advantageously be used also for charging purposes of internal combustion engines. In the operation of such a displacement machine, which operates as a compressor, a plurality of approximately sickle-shaped working chambers are enclosed along spiral delivery spaces between likewise spirally designed bars, which act as displacement bodies, and the two cylinder walls of the delivery spaces, because of the different curvature of the spiral shape, and these working chambers move from an inlet for the operating medium, through the delivery spaces, to an outlet, their volume being continuously reduced and the pressure of the operating medium being correspondingly increased. The two bars, which are in each case arranged on one side of an eccentrically driven disk, are offset by about 180.degree. in relation to each other and extend over approximately 360.degree.. Each bar, viewed in the radial direction, has outer and inner sealing faces which begin at the inlet-side beginning of the bars and end at the outlet-side end of the bars. Each of the bars thus forms, together with the corresponding cylinder wall of the associated delivery space, a working chamber at every 180.degree. angle of rotation of the drive.
Swiss patent number 673 679 also shows in detail the manner in which, in the case of displacement machines based on the spiral principle, the working chambers are produced by a circulating movement of the bars that are fitted to the disk of the displacer, in co-operation with the spiral delivery spaces in the housing. Such working chambers are bounded both by the inside of the bar and by the outside. It is typical of this arrangement that, during the progressive rotary movement of the eccentric drive and guide shafts, a new working chamber is formed and, respectively, the filling operation of a working chamber is completed by a spiral bar approximately every 180.degree. angle of rotation.
In the case of the displacement machine disclosed by DE-A-42 03 346 and Swiss patent number 673 679, having four delivery spaces, there being on each side of the disk two delivery spaces that are offset by about 180.degree. with respect to each other, run spirally from the relevant inlet to the outlet and are arranged with mirror symmetry, during machine operation a filling operation of the operating chambers is completed in each case approximately every 180.degree. angle of rotation of the eccentric drive and guide shafts. The sequence of opening the working chambers toward the central outlet behaves in a manner similar to the filling operation of the working chambers. In the four delivery spaces, as they have been described, the progressive movement of the spiral bars, which are held of the disk of the rotor, means that a total of eight working spaces are formed for each complete revolution of the shafts, in each case four of these working spaces working synchronously with each other. This leads to a low-frequency, pulsating delivery of the operating medium. The remaining irregularity, both of the intake volume flow and of the outgoing volume flow of the operating medium, produces a noise with a basic frequency of twice the speed of rotation of the drive.
Furthermore, in the case of the known displacement machines, the working chambers that are located on the inside and on the outside with reference to a bar have different volumes. This inequality of the working chambers may lead to undesired pulsations in the lines which lead the operating medium to the displacement machine or away from this.
If displacement machines of this type are used for charging internal combustion engines, this low-frequency noise may have a disruptive effect in machine operation. The damping of this noise is associated with additional cost both on the intake side and on the delivery side.
DE-A-41 33 429 discloses one possibility of disrupting the symmetric sequence of the intake and delivery cycles. The solution shown therein is based on displacing the delivery spaces in the housing and the spiral bars on the disk of the rotor in a polar manner with respect to one another in such a way that the sequence of the intake and delivery cycles is no longer uniform. Furthermore, the bars on the disk of the rotor are no longer arranged with mirror symmetry on both sides of the disk, but are again displaced in angular terms with respect to one another in relation to the axis of rotation of the eccentric drive shaft. This further increases the desired irregularity of the intake and delivery cycles of the machine.
However, this solution for smoothing the intake and delivery volume flow of the operating medium is accompanied by the disadvantage of the asymmetry of the reaction forces of the operating medium which act on the bars. In the case of the symmetrical design of the displacement machines according to DE-A-42 03 346 and Swiss patent number 673 679, the reaction forces which act on the bars from the delivery medium during machine operation act symmetrically, and the resulting reaction force lies in the plane of the disk of the rotor. As a result, no forces which would have to be countered by specific precautions are produced. On the other hand, in the case of an asymmetrical arrangement, as is disclosed in DE-A-41 33 429, tilting forces are certainly produced, and are caused both by the unequal loading, brought about by the reaction forces of the operating medium, and as a result of the asymmetrical arrangement of the bars and the therefore non-uniform mass distribution on the disk. The second cause, that of the asymmetrical mass distribution, has a damaging effect in particular at high rotational speeds, since the inertial forces or mass forces are particularly high in such operating conditions.